1. Field of the Invention
This invention relates to a process for forming a wiring on a substrate, and more particularly to a process for forming a wiring on a substrate according to a lift-off technique.
2. Description of the Prior Art
A known process for forming a wiring on a substrate according to the lift-off technique comprises forming a wiring pattern with a lift-off material, then forming a wiring conductor of Al, Cu, Au, or the like on the wiring pattern parts and on the lift-off material by vapor deposition, sputtering, etc., and removing the conductor on parts other than the wiring parts together with the lift-off material, thereby obtaining a wiring of desired pattern. Examples of processes for forming wiring according to the lift-off technique are shown in "Semiconductor International", December, 1981, pages 72-89 and J. Vac. Sci. Technol. B, Vol 1, No. 2, April-June, 1983, pages 490-493. In the process according to the lift-off technique, the lift-off material usually has a reversed tapered form so that a wiring conductor may not be connected to unwanted conductors. Furthermore, the lift-off material must have a good heat resistance because vapor deposition and sputtering of the wiring conductor such as Cu, Al or Au is usually carried out at a high temperature, for example, 200-350.degree. C. to improve the adhesion to a substrate. Still furthermore, the lift-off material must not release a gas, etc. for example, also must be readily removable by lifting off.
Usually, polyimide-based resin is used as the lift-off material, and also photoresists are used frequently. However, these lift-off materials have the following disadvantages.
In the case of a photoresist as the lift-off material, the wiring conductor must be vapor deposited at room temperature or below the room temperature because the photoresist has a very low heat resistance, and also it is hard to form a wiring conductor with good adhesion to a substrate. When the wiring conductor is vapor deposited at 100.degree. C. or higher, the photoresist is deteriorated by the heat and cannot be lifted off even by using an etching solution.
In the case of polyimide-based resin as the lift-off material, there are no problems at all, as long as the lift-off material and the wiring conductor each have a thickness of about 1 .mu.m, whereas there appears a "head-to-lift off" problem, if the thickness is in a range of 10 to 20 .mu.m. Specifically, the polyimide-based, resin is usually lifted off either by O.sub.2 plasma asher or by a etching solution.
Lifting off of the polyimide-based resin by O.sub.2 plasma asher takes a long time with a poor working efficiency, and also it is hard to remove the resin film completely, that is, there remain the resin residues.
Lifting off of the polyimide-based resin by an etching solution has the problem that, when a wiring conductor has a large thickness or when a polyimide-based resin film, that is, the lift-off material, must have a large thickness, it is hard to dissolve the lift-off material, and therefore, it is hard to remove the lift-off material. To solve the problem, the lift-off material must be dipped in the etching solution for a few hours. When a wiring of Al or Cu is dipped in the etching solution for a long time, the wiring part is peeled off.
A wiring conductor having a large thickness, for example, 10-20 .mu.m, is now in a keen demand in a module substrate for mounting a large-scale integration (LSI), and it is necessary to develop a process for readily forming a wiring having such a thickness as above according to the lift-off technique. Prior art on the lift-off technique includes Japanese Patent Applications Kokai (Laid-open) Nos. 54-87525 and 52-156583, where the former discloses a double layer structure of a photoresist and the latter discloses use of polyimide-based resin as a lift-off material.